Thin film capillary process and apparatus

ABSTRACT

Method and system of forming microfluidic capillaries in a variety of substrate materials. A first layer of a material such as silicon dioxide is applied to a channel etched in substrate. A second, sacrificial layer of a material such as a polymer is deposited on the first layer. A third layer which may be of the same material as the first layer is placed on the second layer. The sacrificial layer is removed to form a smooth walled capillary in the substrate.

[0001] The United States Government has rights in this invention pursuant to Contract No. W-7405-ENG-48 between the United States Department of Energy and the University of California for the operation of Lawrence Livermore National Laboratory.

FIELD OF THE INVENTION

[0002] The present invention relates to microfluidic substrates, and more specifically, it relates to the formation of microfluidic capillaries.

BACKGROUND OF THE INVENTION

[0003] Microfluidic technology represents a revolution in laboratory experimentation. Microfluidic circuits allow for scientific experiments that used to be performed at the laboratory bench to be done in volumes as small as a nanoliter. Microfabrication makes it possible to create intricate designs of interconnected channels that are extremely small. Each pattern is designed to produce a series of fluid manipulation steps that will execute an experiment. Hoewever, current capillary electrophoresis on a chip often requires glass to glass or quartz to quartz bonding to form the interconnected channels.

SUMMARY OF THE INVENTION

[0004] Aspects of the present invention include a method comprising: depositing a first layer on a substrate surface; depositing a second layer on the first layer; depositing a third layer on the second layer; and removing the second layer to form a capillary between the first and third layers.

[0005] Further aspects of the present invention include an apparatus comprising: a substrate having a capillary lined with a material from the group consisting of glass, quartz, polysilicon, silicon nitride, silicon dioxide, and boron nitride/silicon dioxide.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] The accompanying drawings, which are incorporated into and form a part of the disclosure,

[0007] FIGS. 1A-1I illustrate a microfluidic device fabricated from a planar substrate.

DETAILED DESCRIPTION OF THE INVENTION

[0008] The embodiments disclosed herein are directed to improved microfluidic devices. Such devices may include a substrate having top, bottom and side surfaces and interconnected fluidic channels within which material is transported during scientific experiments such as electrophoresis. As used herein, the terms “microscale” or “microfabricated” generally refer to structural elements or features of a device which has at least one fabricated dimension in the range of from about 0.1 microns to about 500 microns. Thus, a device referred to as being microfabricated or microscale will include at least one structural element or feature having such a dimension. When used to describe a fluidic element, such as a capillary, channel, passage, or conduit, the terms “microscale,” “microfabricated” or “microfluidic” generally refer to one or more fluid capillaries, channels, passages, or conduits which have at least one internal cross-sectional dimension, e.g., depth, width, length, or diameter that is less than 500 microns, and typically between 0.1 microns and about 500 microns. A typical capillary constructed in accordance with the methods discussed below may have a width and depth in the ranges of approximately 5 to 50 microns and, typically, width and depth in the ranges of approximately 10 to 20 microns.

[0009] FIGS. 1A-1I illustrate a method and system of using a sacrificial layer to form a capillary or channel. In a first step, shown in FIG. 1A, capillary images are first printed on a substrate 100 by means of standard photolithography techniques using a positive photoresist layer 102 and light 104. Substrate 100 may be a silica based substrate from the group consisting of glass, quartz, pyrex, silicon or polysilicon, as well as other substrate materials, such as gallium arsenide. The substrate 100 may also be made of polymeric materials, (e.g., plastics such as polymethylmethacrylate (PMMA), polycarbonate, polytetrafluoroethylene (TEFLON™), polyvinylchloride (PVC), polydimethylsiloxane (PDMS), and polysulfone).

[0010] After development, the photoresist layer 102 is isotropically etched to form a channel 105 as shown in FIG. 1B. Next, the substrate 100 is etched as shown in FIG. 1C to form channel 106 with a predetermined width and depth. This predetermined width may be in the range of 5 to 50 microns and, typically, 10-20 microns. The predetermined depth may also be in the range of 5 to 50 microns and, typically, 10-20 microns. In a next step, the photoresist layer 102 is removed and a first layer 108 is coated over the substrate 100 to line the channel 106, but not fill it as shown by FIG. 1D. The first layer 108 may be applied, for example, by electron sputtering, E-beam coating or chemical vapor deposition and may comprise one of a group consisting of glass, quartz, polysilicon, silicon nitride, silicon dioxide, boron nitride, and various metals. In the disclosed embodiment, the first layer is a silicon dioxide (SiO₂) material with a melting temperature greater than 1000 degrees Celsius.

[0011] A second layer 110 such as a polymer having a high temperature melting point (e.g., wax) is next applied as shown in FIG. 1E to cover the first layer 108 through standard deposition techniques. The melting temperature of the polymer should be approximately in the range of 200 to 300 degrees Celsius, and, typically, approximately 275 degrees Celsius. This second layer 110 is a sacrificial layer or filler. In alternative embodiments, an oxide with a low temperature melting point may also be used for the second layer 110 instead of a high temperature polymer. The melting temperature of the low temperature oxide would be in the range of 200 to 250 degrees Celsius. Because of the nature of the material used in the second layer or filler 110, there is maintained the integrity or smoothness of the first layer that may provide a capillary with a smooth walled surface. The smooth walled surface of the capillary prevents blockage or hindrance of the flow of microfluidic material as it passes through the finished capillary. Typically, the smooth walled surface in the finished capillary will have a roughness approximately in the range of 40 to 60 root mean square (rms), and, typically, approximately 50 rms.

[0012] By repeating the photolithography procedures with the same mask, but negative photoresist (not shown in figures), the filler 110 and first layer 108 is removed from the unmask surface except for the channel region. The substrate 100 is then planarized by reflowing the filler 110 in the channel 106. A chemical mechanical polish (CMP) may then be performed to achieve a flat surface as shown in FIG. 1F.

[0013] Next, a third layer 112 is applied to the substrate 100 which may be a similar or the same material as the first layer 108 and will, therefore, have a melting temperature greater than 1000 degrees Celsius. The third layer is deposited on top of the filler 110 so as to cover the whole surface of the substrate 100 as shown in FIG. 1G and produce an integral mass. The third layer 112 may be applied, for example, by electron sputtering, E-beam coating or chemical vapor deposition. Since the third layer 112 is the same or similar material as the first layer 108, the walls of the capillaries can be uniform in material. In the next step, the filler 110 is removed to form the thin film capillary 114 on the substrate 100 as shown in FIG. 1H. The filler 110 may be removed by heating and melting process.

[0014] Optionally, the substrate 100 may be completely removed to form a stand alone, three dimensional thin film capillary as shown in FIG. 1I.

[0015] The embodiments disclosed herein have a wide variety of uses, including, for example, capillary electrophoresis, liquid chromatograph, miniature chemical factory, polymerase chain reaction (PCR), and other microelectromechanical (MEMS) liquid fluidic gas pumping systems that requires precise injection of liquid and gas samples. Further the embodiments described herein may be used in the performance of high throughput screening assays in drug discovery, immunoassays, diagnostics, nucleic acid analysis, including genetic analysis, and the like. As such, the embodiments described herein will often include multiple sample introduction ports or reservoirs, for the parallel or serial introduction and analysis of multiple samples.

[0016] The foregoing is illustrative of the present invention and is not to be construed as limiting thereof. The invention is defined by the following claims, with equivalents of the claims to be included therein 

1. A method comprising: depositing a layer on a substrate surface; and removing said layer to form a capillary in said substrate surface.
 2. The method of claim 1, wherein said layer is a polymer or oxide material.
 3. A method comprising: depositing a first layer on a substrate surface; depositing a second layer on said first layer; depositing a third layer on said second layer; and removing said second layer to form a capillary between said first and third layers.
 4. The method of claim 3, wherein said first layer is from a group consisting of glass, quartz, polysilicon, silicon nitride, silicon dioxide, and boron nitride.
 5. The method of claim 3, wherein said first layer is silicon dioxide.
 6. The method of claim 3, wherein said substrate is a silica based material.
 7. The method of claim 3, wherein said substrate is from a group consisting of glass, quartz, pyrex, silicon and polysilicon.
 8. The method of claim 3, wherein said substrate is a polymeric based material.
 9. The method of claim 3, wherein said substrate is from the group consisting of polymethylmethacrylate (PMMA), polycarbonate, polytetrafluoroethylene (TEFLON™), polyvinylchloride (PVC), polydimethylsiloxane (PDMS), and polysulfone.
 10. The method of claim 3, wherein said second layer is a polymer material.
 11. The method of claim 3, wherein said second layer is an oxide material.
 12. The method of claim 3, wherein said first and third layers are the same material.
 13. The method of claim 3, further comprising: removing all of said substrate except a portion of said substrate surrounding said capillary.
 14. A method comprising: etching a channel in substrate surface; depositing a first layer on said channel; and depositing a second layer on said first layer.
 15. The method of claim 14, further comprising: depositing a third layer on said second layer; and removing said second layer to form a capillary between said first and third layers.
 16. The method of claim 15, wherein said second layer is melted during the removing step.
 17. A method of forming a capillary in a microfluidic device comprising: depositing a first layer on a substrate surface; depositing a second layer on said first layer; depositing a third layer on said second layer; and removing said second layer to form a capillary between said first and third layers.
 18. The method of claim 17, further comprising: reflowing said second layer.
 19. A method of forming a thin film capillary in a microfluidic device comprising: performing a first photolithographic process using a mask and a positive photoresist to develop a capillary image onto a substrate surface; etching a channel in said substrate corresponding to the capillary image; depositing a first silicon dioxide layer on said substrate surface; depositing a wax filler layer on said first silicon dioxide layer; performing a second photolithograpic process using said mask and a negative photoresist to develop an image opposite to the capillary image onto the substrate surface; etching the silicon dioxide and the wax filler layers from the substrate surface not covered by the mask; depositing a second silicon dioxide layer on said substrate surface; and melting said wax filler layer.
 20. The method of claim 3, wherein said first layer has a melting point greater than approximately 1000 degrees Celsius.
 21. The method of claim 20, wherein said third layer has a melting point greater than approximately 1000 degrees Celsius.
 22. The method of claim 3, wherein said second layer has a melting point in the range of approximately 200 to 300 degrees Celsius.
 23. The method of claim 3, wherein said second layer has a melting point in the range of approximately 200 to 250 degrees Celsius.
 24. The method of claim 3, wherein the first and third layers have melting points greater than approximately 1000 degrees Celsius and the second layer has a melting point in the range of approximately 200 to 250 degrees Celsius.
 25. An apparatus comprising: a substrate having a capillary lined with a material from the group consisting of glass, quartz, polysilicon, silicon nitride, silicon dioxide, and boron nitride/silicon dioxide.
 26. The apparatus of claim 25, wherein said substrate is from a group consisting of glass, quartz, pyrex, silicon and polysilicon.
 27. An apparatus comprising: a glass substrate having a capillary lined with silicon dioxide.
 28. An apparatus comprising: a substrate having a capillary lined with a material from the group consisting of glass, quartz, polysilicon, silicon nitride, silicon dioxide, and boron nitride; and said capillary filled with material from the group consisting of a polymer and an oxide.
 29. The apparatus of claim 28, wherein said capillary is filled with a polymer.
 30. The apparatus of claim 29, wherein said polymer is wax.
 31. An apparatus comprising: a substrate having a capillary lined with a material having a melting point greater than approximately 1000 degrees Celsius.
 32. The apparatus of claim 31, a substrate having a capillary lined with a material; and said material having a melting point greater than approximately 1000 degrees Celsius.
 33. The apparatus of claim 31, wherein said capillary has a wall roughness of approximately 50 rms.
 34. The apparatus of claim 31, wherein said substrate is a single, integrated structure.
 35. A microfluidic device comprising: a glass substrate having a capillary lined with silicon dioxide; and said capillary filled with a polymer or oxide.
 36. A microfluidic device comprising: a substrate having a smooth walled capillary lined with a material from the group consisting of glass, quartz, polysilicon, silicon nitride, silicon dioxide, and boron nitride silicon dioxide.
 37. A microfluidic device comprising: a substrate having a capillary lined with a material; and said material having a melting point greater than approximately 1000 degrees Celsius.
 38. A microfluidic device comprising: an integrated substrate having a capillary lined with a material; and said material having a melting point greater than approximately 1000 degrees Celsius.
 39. A device made by the method of claim
 1. 40. A device made by the method of claim
 3. 41. A device made by the method of claim
 14. 42. A microfluidic device made by the method of claim
 1. 43. A microfluidic device made by the method of claim
 3. 44. A microfluidic device made by the method of claim
 14. 